Federal externship would probably give you good experience and lots to talk about. A small or mid-sized firm would come next.
If you don't get a paid job, I'd consider finding a PT research position for some cash. I worked a FT job at the DA's office last summer, and then an additional 10-15 hours with a professor. It kind of sucked, but I made a little cash, and now have a great relationship with a professor who will recommend me for anything. If you have any interest in clerking, these faculty contacts are an absolute necessity.
If you're choosing between them though, I'd put the RA position as a last choice among the others.